This invention relates to an optical signal processor, and more specifically, to an optical signal processor for processing a signal in an optical state in an optical network system, an optical switching system and so on.
As an infrastructure for supporting the future information scene, a large capacity optical communication network using a wavelength division multiplexing system has been eagerly researched. In such present situation, a practical optical reproduction technology such as signal wavelength conversion and waveform shaping has been positively studied and examined, since the capacity of an optical network greatly improves if it is possible to reproduce a signal in an optical state at each node of the optical network.
In order to obtain such optical reproducer, the following configurations have been basically proposed; a configuration (cf. FIGS. 11 and 12 of the U.S. Pat. No. 5,959,764) employing the cross gain modulation and cross phase modulation of a semiconductor optical amplifier (SOA), a configuration (cf. e.g. FIG. 13 of the above-mentioned patent) utilizing four wave mixing of a semiconductor optical amplifier or an optical fiber, and a configuration (cf. e.g. FIG. 1 of the above-mentioned patent) using cross gain modulation of an electroabsorption (EA) optical modulator. Also, a configuration in which a semiconductor optical amplifier is disposed at each of two optical paths or arms of an interferometer configuration is disclosed in the U.S. Pat. Nos. 6,005,708 and 6,035,078 and EP 0717482.
In the foregoing conventional art, the one using the cross gain modulation of the semiconductor optical amplifier (SOA) has a simple configuration. However, this type is not suitable for multistage wavelength conversion since it is unable to produce converted light having a sufficiently high extinction rate. Furthermore, the gain recovery time takes several hundreds picoseconds and so it is not applicable to a high-speed signal of several Gb/s or more because the pattern effect becomes prominent.
The configuration, in which the two semiconductor optical amplifiers are disposed to compose a Mach-Zehnder interferometer structure, becomes complicated since original signal light must be input differentially when the high-speed is pursued. Also, this type has another problem that converted light is sensitively affected by only a slight power fluctuation of the original signal light since phase modulation of 180xc2x0 can be performed with relatively low optical power.
Although the one utilizing the four wave mixing of a semiconductor optical amplifier operates at high-speed, its conversion wavelength band is narrow and S/N ratio is deteriorated due to spontaneous emission. When the four wave mixing is used, it is necessary, in principle, to equalize a polarization plane of original signal light with that of pumping light. The configuration of the surrounding optical elements, thus, becomes complicated causing the high production costs.
Although the wavelength converter employing the cross gain modulation of an electroabsorption optical modulator has the simplest configuration, this type demands relatively large original signal input light.
An object of the present invention is to solve the aforesaid problems in the conventional art and provide an optical signal processor for efficiently reproducing (wavelength converting and/or waveform shaping) a signal with a simple configuration.
Another object of the present invention is to provide an optical signal processor capable of obtaining a larger extinction rate.
Further object of the present invention is to provide an optical signal processor capable of shaping a signal waveform in an optical state.
An optical signal processor according to the invention is composed of a first optical path having an electroabsorption optical modulator which is applied with a constant voltage and absorbs light of a signal wavelength, a second optical path having a fixed phase relation with the first optical path relative to a probe wavelength, a probe light introducer for dividing probe light of the prove wavelength into two portions and feeding them respectively into the first and second optical paths, an original signal light introducer for introducing original signal light of the signal wavelength into the first electroabsorption optical modulator, and a combiner for combining both light of the probe wavelength passed through the first and second optical paths.
With this configuration, cross phase modulation occurs between the original signal light and the probe light in the first electroabsorption optical modulator, and the probe light is phase-modulated. Consequently, by combining the probe light propagated on the first and second optical paths, light (converted signal light) of the probe wavelength can be obtained which waveform varies at high-speed by following a waveform variation of the original signal light. Since the constant voltage is applied to the electroabsorption optical modulator, a carrier generated by the original signal light can be discharged toward the outside at high-speed. Hence, a high-speed response as fast as 10 Gb/s and more can be realized.
In the cross phase modulation of the electroabsorption optical modulator, since the intensity variation greater than that of the original signal light can be given to the combined probe light, the extinction rate is improved. Also, it is possible to give compression characteristics to the low intensity part and high intensity part of the original signal light, and therefore the noise can be suppressed and so the waveform is improved to have a steeper shape.
Preferably, the original signal light introducer introduces the original signal light into the first optical path so as to propagate in the opposite direction to the probe light in the first electroabsorption optical modulator. This configuration prevents that the remainder of the original signal light passed through the first electroabsorption optical modulator is mixed with the output light of the optical signal processor.
Preferably, one of the first and second optical paths has a phase shifter for adjusting the phase difference between the first and second optical paths relative to the probe wavelength into a predetermined value. With this configuration, desired characteristics can be obtained easily, and also an inverter operation can be selected.
Preferably, the second optical path has a second electroabsorption optical modulator having the same characteristics with those of the first electroabsorption optical modulator. With this configuration, it becomes easy to adjust or set the phase relation between the first and second optical paths.
An optical signal processor according to the invention is also composed of a first optical path having a first electroabsorption optical modulator, which is applied by a constant voltage and absorbs light of a signal wavelength, and a first reflector for reflecting a probe wavelength on one end of the first optical path, a second optical path having a fixed phase relation with the first optical path relative to the probe wavelength and having a second reflector for reflecting the probe wavelength on one end of the second optical path, an original signal light introducer for introducing original signal light of the signal wavelength into the first electroabsorption optical modulator, a combiner/divider for dividing probe light of the probe wavelength into two portions and feeding them respectively into the first and second optical paths as well as combining the two portions of light from the first and second optical paths, and a probe wavelength extracting filter for extracting the optical component of the probe wavelength from the combined output light of the combiner/divider.
With this configuration, it is also possible to obtain the same operation effect as a Michelson interferometer optical circuit.
Preferably, the original signal light introducer introduces the original signal light into the first optical path so as to enter the first electroabsorption optical modulator in the same direction with the probe light. With this configuration, the original signal light is absorbed while making roundtrips in the first electroabsorption optical modulator. Consequently, since the light intensity of the remained original signal light becomes considerably weak, harmful effects such as interference decrease.
Preferably, one of the first and second optical paths has a phase shifter for adjusting the phase difference between the first and second optical paths relative to the probe wavelength into a predetermined value. This configuration makes it possible to easily obtain desired characteristics. Also, an inverter operation can be selected.
Preferably, the first reflector is formed on one end face of the first electroabsorption optical modulator. This configuration makes it easier to produce and adjust the optical signal processor.
Preferably, the second optical path has a second electroabsorption optical modulator having the same characteristics with those of the first electroabsorption optical modulator. This configuration makes it easier to adjust or set the phase relation between the first and second optical paths.
Preferably, the second reflector is formed on one end face of the second electroabsorption optical modulator. This configuration makes it easier to produce and adjust the optical signal processor.